1. Field of the Invention
The present invention generally relates to a microwave circuit, and more particularly to a dual-band active filter having positive gain and bandpass spectral performance for microwave and millimeter-wave wireless communication systems.
2. Description of the Related Art
The rapid development of the coexisted operation of multi-standard wireless and mobile communication has been driving conventional RF and baseband transceivers to have integrated multi-band and multi-functional characteristics, such as the multimode wireless LAN IEEE802.11a/b/g PCMCIA card, the integrated Bluetooth/WLAN card, and the integrated GSM/WLAN handset. This requirement has driven the conventional single-band RF circuits, such as the low-noise amplifier (LNA), bandpass filters (BPF), mixers, voltage-controlled oscillators (VCOs) and the power amplifier (PA), to a new design era. Moreover, the fast developing radio-on-a-chip circuit contains the monolithic active chips, but still needs off-chip passive filters to achieve maximum performance and lowest cost. But the off-chip passive filters, conventionally implemented by waveguide or planar structures on ceramic substrates, are bulky or inherently lossy that they are difficult in achieving true single-chip implementation. A variety of conventional active filter design methods offer good solution of this difficulty, but they are limited to single-band operation.
To achieve dual-band or multi-band operation, the paper, reported by Quend et al entitled “An original topology of dual-band filter with transmission zeros,” IEEE MTT-S Technical Digest 2003, vol. 2, pp. 1093-1096, provided a dual-band design based on the parallel combination of multiple dual-behavior resonators. Another approach proposed by the inventor and co-workers employs the step-impedance resonator to obtain dual passbands with high stop-band suppression, as detailed in Electronics Letters, vol. 50, pp. 38-39, 2004. Although these methods achieve good dual-band filter performance, they belong to a passive structure due to their use of microstrip line structures. They provide no signal gain and, in reality, they have loss due to the substrate and conductor losses.
U.S. Pat. No. 5,995,814 issued to James Yeh., entitled “Single-stage dual-band low-noise amplifier for use in a wireless communication system receiver”, discloses that a low-noise amplifier capable of operating in two frequency bands for Personal Communication Services (PCS) receiver or other wireless communication system terminals. An exemplary embodiment of the amplifier includes a single bipolar-junction-transistor amplification stage, where the input and output matching networks for each transistor include a first series capacitor, a shunt inductor coupled to ground potential, a second series capacitor and a second inductor coupled to a supply voltage. These four elements are used to provide narrowband input or output matches in two distinct frequency bands, such as 900 MHz and 1.8 GHz. The matching networks can be realized by using only lumped inductive and capacitive elements. The amplifier may be configured to provide a minimum gain of 15 dB and a maximum noise figure of 2 dB over 100 MHz-wide bands centered at 900 MHz and 1.8 GHz. Although this dual-band LNA provides dual-band gain, it suffers poor stop-band suppression. Thus it needs to connect with extra passive filters to have strong out-of-band interference suppression. According to the above problems, there is a need to provide a dual-band active filter having the positive gain and filter performance to overcome the conventional disadvantages.